The present invention relates generally to electronic dimming ballasts for gas discharge lamps. More particularly, the present invention pertains to methods and circuits used in electronic ballasts for maintaining stable lamp operation when dimming the lamp.
Dimming ballasts control light output from a light fixture while saving energy. An exemplary prior art series resonant inverter ballast for a fluorescent lamp is shown in FIG. 5. In FIG. 5, a typical single lamp dimming ballast topology is based on a series resonant inverter including a resonant inductor L1 and a resonant capacitor C1 that form a resonant tank. A first alternating current (AC) source VAC1 represents the equivalent output of a half-bridge inverter configured to drive the resonant tank. The first AC source VAC1 provides a frequency controlled fixed voltage input for the resonant tank. By changing the frequency of the first AC source VAC1, the current provided by the resonant tank to the lamp L2 can be adjusted.
A second AC source VAC2 is a fixed voltage, variable frequency AC source used to drive a filament heating circuit. A filament capacitor C3 and a filament heating transformer primary winding T1 form a filament heating resonant tank that changes the power provided to the filaments of the lamp is response to variation in the frequency of the second AC source VAC2. The filament heating transformer has a first secondary winding T2 and a second secondary winding T3. The first secondary winding T2 and the second secondary winding T3 drive a first lamp filament RF1 and a second lamp filament RF2 respectively. A microcontroller 504 receives and decodes a dimming control signal and adjusts the frequency of the first AC source VAC1 and the second AC source VAC2 to adjust the lamp current and the filament heating current.
Dimming ballasts, however, generally cannot provide a stable current to a lamp when the ambient temperature is low (e.g., less than 10 degrees C.). The primary reason for instability of the lamp current while the ballast is reducing current to correspond to a selected dimming level is that the impedance of the fluorescent lamp changes suddenly at low ambient temperatures when dimming to a selected dimming level. Because this unstable lamp impedance is unpredictable, it is difficult to design a control loop to stabilize the lamp current and light output. FIGS. 1-4 graphically show a series of T528 W lamp currents when dimming the lamp from 72 mA to 20 mA at 5 degrees C. ambient temperature. As shown in FIG. 1, lamp current is stable when lamp current is around 72 mA. Referring to FIG. 2 which shows lamp current at 56 mA and FIG. 3 which shows lamp current at 36 mA, the lamp current starts to fluctuate when the lamp current is reduced to between 56 mA and 20 mA in a 5 degrees C. ambient temperature environment. The lamp current stabilizes when it gets down to around 20 mA as shown in FIG. 4.
The unstable lamp impedance and current can cause visible lamp flickering when the ballast is dimming the lamp to a selected light output level (i.e., selected dimming level). Because lamp impedance is very difficult to predict under low current and low temperature conditions, lamp flickering cannot be easily eliminated with conventional lamp current control loop designs. For this reason, dimming ballasts available today generally list a minimum ambient operating temperature requirement for normal lamp operation of, for example, 0 degrees C. for supplying current to a T8 type lamp and 10 degrees C. for supplying current to a T5 type lamp.